Metal organic framework-loaded polyethersulfone/polyacrylonitrile photocatalytic nanofibrous membranes under visible light irradiation for the removal of Cr(vi) and phenol from water

In this work, various amounts of the UiO-66-NH2 and UiO-66-NH2/TiO2 MOFs have been loaded into polyacrylonitrile (PAN) nanofibers supported on polyethersulfone (PES). The visible light irradiation was used to investigate the influence of pH (2–10), initial concentration (10–500 mg L−1), and time (5–240 min) on the removal efficiency of phenol and Cr(vi) in the presence of MOFs. The reaction time: 120 min, catalyst dosage: 0.5 g L−1, pH: 2 for Cr(vi) ions and pH: 3 for phenol molecules were optimum to degrade phenol and to reduce Cr(vi) ions. The characterization of the produced samples was performed using X-ray diffraction, ultraviolet-visible diffuse reflectance spectroscopy, scanning electron microscopy, and Brunauer–Emmett–Teller analysis. The capability of synthesized photocatalytic membranes was investigated for the removal of phenol and Cr(vi) ions from water. The water flux, Cr(vi) and phenol solutions fluxes and their rejection percentages were evaluated under pressure of 2 bar in the presence of visible light irradiation and in the dark. The best performance of the synthesized nanofibers was obtained for UiO-66-NH2/TiO2 MOF 5 wt% loaded-PES/PAN nanofibrous membranes at temperature of 25 °C and pH of 3. Results demonstrated the high capability of MOFs-loaded nanofibrous membranes for the removal of various contaminants such as Cr(vi) ions and phenol molecules from water.


Introduction
The rapid development of industry and shortage of water resources caused the development of novel alternative faster methods for the rapid removal of contaminants from water. 1 Various technologies including advanced oxidation processes (AOPs), membrane separation, coagulation, adsorption, and ion exchange have been used to remove toxic matters from water. 2 Recently, hybrid methods such as adsorption/photocatalysis, [3][4][5] coagulation/adsorption 6,7 and photocatalysis/membrane [8][9][10][11] have been developed to increase the removal efficiency of effluents and accelerate their treatment compared with simple treatment techniques. The photocatalysis/membrane technique is a physical separation/chemical oxidation combined method for the reduction of membrane fouling and increasing the removal efficiency of membranes. 10 Metal organic frameworks (MOFs) as novel photocatalysts have been utilized for degrading organic effluents and reducing metal ions, due to their adjustable pores, high surface area, and high photocatalytic activity through the charge transfer between organic ligand-metal cluster under visible light irradiation. [12][13][14][15][16] MOFs used for photo-degradation of toxic matters from aquatic systems include various types of UiO, MIL, and ZIF. 17 However, the use of pure MOFs due to difficult recycling aer the photocatalysis process is limited. [18][19][20] The MOFs loaded membranes and development of photocatalytic membranes is an effective method for (I) uniform disposition of MOFs on the support, (II) use of MOFs in large-scale experiments, (III) prevention of their agglomeration during the photocatalysis process, and (IV) easier recycling aer removal of effluents. 21 For instance, Du et al. 22 investigated the performance of UiO-66-NH 2 membrane supported on a-Al 2 O 3 under sunlight irradiation for reduction of Cr(VI) ions. Liu et al. 23 incorporated the Ni@UiO-66 MOFs into the polyethersulfone (PES) membrane under UV irradiation for the water treatment. Ahmadi et al. 24 immobilized 0.2 wt% NH 2 -MIL125(Ti) MOF on the polysulfone membrane for photodegradation of methylene blue under UV irradiation. They also suspended the MOF nanoparticles in the reactor. The methylene blue removal efficiency and ux recovery ratio were 97% and 88%, respectively. Sun et al. 25 incorporated the poly(sulfobetaine methacrylate)/UiO-66 composite into the polysulfone ultraltration membrane. The water ux of the MOFbased composite-incorporated polysulfone was higher than that of the polysulfone membrane (about 2.5 times). Salehian et al. 26 investigated the removal efficiency of natural organic matter using a TiO 2 @MIL-88A (Fe)-loaded polyacrylonitrile photocatalytic membrane. The humic acid removal efficiency and ux recovery ratio of the membrane were 92.4% and 99.5%, respectively. The nanobers prepared by electrospinning are good candidates for incorporating MOFs. 27,28 In recent years, the nanobrous mats have been extensively utilized as a membrane in ultraltration, microltration, nanoltration and forward osmosis membrane processes. [29][30][31][32] However, the use of nano-bers in the continuous wastewater treatments such as membrane processes due to their low mechanical stability is limited. For instance, Khalil et al. 33 investigated the potential of PAN/SiO 2 -TiO 2 -NH 2 composite nanobers for degradation of acid red 27 and malachite green under visible light. The rapid degradation of acid red 27 and malachite green using nano-bers was occurred during 9 and 25 min, respectively. In another study, the performance of a SiO 2 -TiO 2 -loaded polyaniline nanober membrane was studied to degrade the methyl orange. 34 The prepared PAN/Ag-TiO 2 nanober membrane indicated the high photocatalytic activity for the complete removal of methylene blue within 1 h. 35 Pu et al. 36 investigated the degradation of ciprooxacin using a PAN/ZIF-65 MOFs nanober membrane. However, there is no study on the removal of phenol and Cr(VI) using polyethersulfone (PES)/PAN/ UiO-66-NH 2 /TiO 2 MOFs nanober membranes. In this work, the synthesized UiO-66-NH 2 /TiO 2 MOFs were rst loaded into the polyacrylonitrile (PAN) solution. The PAN/MOFs have been electrospun on the PES nanobrous support to prepare the PES/ PAN/MOFs photocatalytic nanobrous membranes. The capability of synthesized photocatalytic membrane was investigated for the removal of phenol and Cr(VI) ions from water under visible light irradiation.

Synthesis of MOFs
UiO-66-NH 2 and TiO 2 nanoparticles were synthesized using hydrothermal and sol-gel methods as described previously. 19,37 To prepare UiO-66-NH 2 /TiO 2 composites, rst 50 mg TiO 2 nanoparticles were dispersed in ethanol. Then, 50 mg UiO-66-NH 2 was dispersed in solution under sonication for 30 min. Aer that, the synthesized hybrid was ltered and washed three times with water and ethanol. Finally, the produced solid was dried at 100°C overnight.

Fabrication of PES/PAN/MOFs membrane
The PES nanobrous support was prepared by electrospinning method by dissolving 2 g PES in 8 mL DMF and its electrospinning under feeding rate of 1 mL h −1 , voltage of 20 kV, and distance of 15 cm. PAN solution was prepared by its dissolving in DMF at 60°C within 4 h. To prepare the PAN/MOFs and PAN/ MOFs/TiO 2 solutions, different amounts of MOFs and MOFs/ TiO 2 (2, 5 and 10 wt% by weight of PAN) were dispersed in DMF. Then, PAN was added under stirring overnight. First, the prepared PAN/MOFs and PAN/MOFs/TiO 2 solutions were sonicated for 30 min, and then were electrospun on the PES support.

Photocatalytic membrane experiments
The performance of the PES/PAN/MOFs nanobrous membranes was examined in a cross-ow photocatalytic membrane reactor under visible light (Xenon arc lamp), operating pressure of 2 bar, effective surface area of 35 cm 2 , and temperature of 25°C. The ltration was carried out for 120 min with an initial feed concentration of 10 mg L −1 . The membranes were regenerated using 0.1 M HCl solution (200 mL) for 2 h. 19 The experimental set-up of the photocatalytic membrane process is illustrated in Scheme 1.
Scheme 1 Experimental set-up of photocatalytic membrane process.

Characterization tests
The morphology of membranes was detected by employing a scanning electron microscopy (SEM) using, JEOL JSM-6380 microscope. An Image J soware (Image-Proplus, Media Cybrernetics) was used to determine the particle size and the size distribution of particles and nanobers. A diffuse reectance spectrum (DRS) of MOFs was recorded using UV-2550 (Shimadzu, Japan) UV-vis spectrophotometer. The crystallinity and surface area of synthesized MOFS were determined using Xray diffractometer type Philips PW 1730 (Japan) and Brunauer-Emmett-Teller (BET) analysis. The contact angle of membranes was investigated using a contact angle meter (CA-VP, Kyowa Interface Science Co., Ltd, Japan). The pore radius (r m ) of nanobrous membranes is calculated as follows: where h is the water viscosity (8.9 × 10 −4 Pa s), Q is the volume of the permeate pure water per unit time (m 3 S −1 ), DP is the operating pressure (0.2 MPa), A is the membrane effective area (m 2 ), l is the thickness of the membrane (m) and 3 is the porosity of the membrane which is dened as follows: 3 ¼ where W 1 is the weight of the wet membrane, W 2 is the weight of the dry membrane and d w is the water density (0.998 g cm −3 ).

Characterization of nanobrous membranes
The SEM images of the surface of PES nanobrous support and PAN nanobrous membranes with different content of MOFs (0, 2, 5 and 10 wt%) are presented in Fig. 2. The homogeneous nanobers with average diameters of 360 ± 60 nm and 250 ± 50 nm were obtained for pure PES (Fig. 2a) and PAN ( Fig. 2b) nanobers, respectively. By loading 5 wt% UiO-66-NH 2 , some MOFs were observed on the nanobers surface and the average diameter of nanobers was increased to 330 ± 120 nm (Fig. 2c).
The similar morphology with an average diameter of 315 ± 100 nm was obtained for 5 wt% UiO-66-NH 2 /TiO 2 -loaded PAN nanobers (Fig. 2e). By loading 2 wt% UiO-66-NH 2 /TiO 2 MOFs, the thinner bers with average diameter of 280 ± 60 nm have been prepared and the most of MOFs nanoparticles without aggregation have been successfully incorporated into the nanobers (Fig. 2d). By loading 2 wt% UiO-66-NH 2 /TiO 2 MOFs into the PAN nanobers, the viscosity of electrospinning solution was increased which resulted in gradual increase in the ber diameter of PAN/UiO-66-NH 2 /TiO 2 2 wt% (280 ± 60 nm) compared to pure PAN nanobers (250 ± 50 nm). By increasing the concentration of UiO-66-NH 2 /TiO 2 in the PAN solution, the aggregation of UiO-66-NH 2 /TiO 2 nanoparticles in the solution and non-homogenous dispersion of nanoparticles resulted in the formation of UiO-66-NH 2 /TiO 2 nanoparticles on the surface of the nanobers. By loading 10 wt% UiO-66-NH 2 /TiO 2 MOFs, most of MOFs were aggregated on the nanobers surface (Fig. 2f) up to 5% into the nanobrous membrane, the porosity and pore size of nanobers was gradually increased and a further increase in the UiO-66-NH 2 /TiO 2 content (10 wt%) resulted in decreasing the porosity and pore size of nanobrous membranes. The increase in the porosity of nanobers by loading of UiO-66-NH 2 /TiO 2 could be attributed to the higher porosity of UiO-66-NH 2 /TiO 2 in the nanobers. The decrease in the porosity and pore size of nanobers containing 10 wt% UiO-66-NH 2 /TiO 2 could be attributed to the nanoparticles aggregation and there are not enough free voids to equilaterally distribute the nanoparticles into the nanobers, as conrmed by SEM image.

Photocatalytic removal of Cr(VI) and phenol in a batch system
The UV diffuse reectance spectra (DRS) of synthesized MOFs are illustrated in Fig. 3 The effect of pH on the photo-degradation of phenol and Cr(VI) using MOFs under visible light, catalyst dosage of 0.5 g L −1 , initial concentration of 10 mg L −1 , reaction time of 240 min, temperature of 25°C, and pH values ranging from 2-10 is illustrated in Fig. 3b. As shown, the maximum removal of Cr(VI) using UiO-66-NH 2 , and UiO-66-NH 2 /TiO 2 MOFs was occurred at pH 2. At lower pH values, the better reduction of Cr 2 O 7 2− ions was occurred, due to the better electrostatic attraction of Cr(VI) anions and synthesized MOFs. Aer that, the removal of Cr(VI) ions was occurred by irradiation of visible light on the MOFs surface via the photogenerated-electron-hole pairs (eqn (3) and (4)). At higher pH values, the precipitation of chromium anions in the form of Cr(OH) 3 might cove the active sites of synthesized photocatalysts and reduced their photocatalytic efficiency (eqn (5)).
The optimum pH for the removal of phenol using synthesized photocatalysts was occurred at pH 3. As shown, the complete degradation of phenol was obtained using UiO-66-NH 2 /TiO 2 MOFs at pH 3 aer 120 min. The maximum phenol removal percentages in the presence UiO-66-NH 2 , and UiO-66-NH 2 /TiO 2 MOFs were 81.3% and 99.5%, respectively. Therefore, the pH values of 2 and 3 were selected for further experiments.

Photocatalytic membranes
The water permeation, Cr(VI) solution ux and phenol solution ux were evaluated at the pressure of 2 bar under visible light irradiation and without light irradiation (Fig. 4). As shown in Fig. 4a,   respectively. By increasing the concentration of UiO-66-NH 2 / TiO 2 , the hydrophilicity of membrane was increased which resulted in increasing the water permeability. The water contact angle of pure PES/PAN, PES/PAN/UiO-66-NH 2 /TiO 2 2%, PES/ PAN/UiO-66-NH 2 /TiO 2 5% and PES/PAN/UiO-66-NH 2 /TiO 2 10% nanobrous membranes were found to be 77.3 ± 1.2°, 65.6 ± 1.4°, 49.8 ± 1.3°, and 38.8 ± 1.2°, respectively. The enrichment of the surface of membranes with -NH 2 and Ti-O groups, resulted in decreasing of the water contact angle and increasing the hydrophilicity of membranes by increasing UiO-66-NH 2 / TiO 2 concentration in the PES/PAN membrane. Furthermore, the loading of UiO-66-NH 2 /TiO 2 with high porosity into the membrane may be increased the membrane porosity and enhanced the water permeability of PES/PAN nanobrous membrane. The light irradiation did not impact on the water permeability. This behavior indicated no signicant reaction between the hydroxyl radicals and polymer chains. Therefore, the intrinsic resistance of the membrane exhibited a critical role on the water permeability. The blocking of some pores of nanobrous membranes with phenol and Cr(VI) resulted in a gradual decrease of phenol and Cr(VI) solutions compared with the water permeability of PES/PAN/UiO-66-NH 2 /TiO 2 MOFs nanobrous membranes (Fig. 4b and c). The Cr(VI) and phenol solutions uxes have been increased in the presence of visible light irradiation. The photocatalytic degradation of Cr(VI) ions and phenol molecules that blocked the nanobers pores, resulted in improving the Cr(VI) and phenol solutions permeability under visible light. The gradual enhancement of Cr(VI) rejection by increasing the concentration of UiO-66-NH 2 /TiO 2 was due to the increasing the hydrophilicity under dark state (Fig. 4d). However, the rejection of phenol did not signicantly change by loading of UiO-66-NH 2 /TiO 2 (Fig. 4e). The photodegradation of phenol and Cr(VI) ions by hydroxyl radicals resulted in increasing removal efficiencies of phenol and Cr(VI) under visible light (Fig. 4d and  e). The removal efficiencies of phenol and Cr(VI) using PES/PAN/ UiO-66-NH 2 /TiO 2 5% were 84.9 and 77.3% under dark state. Whereas, the maximum removal efficiencies of phenol and Cr(VI) were 92.7 and 96.3% in the presence of PES/PAN/UiO-66-NH 2 /TiO 2 5% nanobrous membrane under visible light. The gradual decrease in the phenol and Cr(VI) rejection percentages by increasing UiO-66-NH 2 /TiO 2 concentration up to 10 wt% may be attributed to the increase in the membrane porosity and pore radius. Similar trend is reported by Ahmadipouya et al.. 39 They found that the mixed-matrix membrane containing 9 wt% UiO-66 was optimum for the removal of dyes and further loading of UiO-66 MOFs (12 wt%) resulted in decreasing the rejection percentages of dyes.
The phenol solution ux, Cr(VI) solution ux, phenol rejection and Cr(VI) rejection during 120 min in the presence visible light irradiation and without light irradiation are presented in Fig. 5. The uxes of phenol and Cr(VI) have decreased from 824.3 L m −2 h −1 bar −1 to 529.6 and 633.1 to 412.3 L m −2 h −1 bar −1 for phenol and Cr(VI) ions solutions using PES/PAN/UiO-66-NH 2 /TiO 2 5% nanobrous membrane in the dark state. The higher hydrophilicity of nanobrous membrane containing 5 wt% UiO-66-NH 2 /TiO 2 compared to the hydrophilicity of composite membranes containing lower amounts of UiO-66-NH 2 /TiO 2 resulted in its lower ux decline. At higher amounts of UiO-66-NH 2 /TiO 2 , the interaction between contaminants and membrane surface resulted in its garadual higher ux decline compared with 10 wt% UiO-66-NH 2 /TiO 2 loaded-PES/PAN nanobrous membrane. In the presence visible light, the ux decline has been improved and the minimum ux decline was found to be 26.0% and 25.8% for Cr(VI) and phenol solutions using PES/PAN/UiO-66-NH 2 /TiO 2 5% nanobrous membrane. The hydroxyl radicals generated during photocatalytic reaction could degrade the phenol molecules and Cr(VI) ions and could prevent the ux decline.
The maximum Cr(VI) and phenol rejection percentages were 84.9 and 77.3% under dark state using PES/PAN/UiO-66-NH 2 / TiO 2 5% which were due to the adsorption of contaminants by the membrane and a further removal of Cr(VI) and phenol under visible light (phenol: 92.7% and Cr(VI) 96.3%) were due to the photocatalytic reduction of contaminants. Therefore, the prepared nanobrous membranes could eliminate Cr(VI) and phenol from water through the adsorption, ltration, and photocatalytic reduction. For the phenol degradation, the removal efficiency did not signicantly change by increasing UiO-66-NH 2 /TiO 2 concentration under the dark state. However, the degradation ability of PES/PAN/UiO-66-NH 2 /TiO 2 was enhanced by increasing UiO-66-NH 2 /TiO 2 content up to 5%, which due to the enhanced photocatalytic capacity of PES/PAN nanobers. Therefore, UiO-66-NH 2 /TiO 2 as a photocatalysis composite could improve the performance of PES/PAN/UiO-66-NH 2 /TiO 2 nanobrous membrane to degrade the phenol molecules. For Cr(VI) reduction, the removal efficiency of PES/ PAN/UiO-66-NH 2 /TiO 2 was increased by loading UiO-66-NH 2 / TiO 2 into the membrane up to 5% under both dark state and visible light irradiation. Therefore, the adsorption capacity, and photocatalytic reduction of membrane have been improved for reducing Cr(VI) ions from water. The obtained results indicated that the prepared photocatalytic membrane exhibited a better photocatalytic performance to eliminate Cr(VI) and phenol under visible light irradiation.
The change in the equilibrium uxes aer regeneration of nanobrous membranes with 0.1 M HCl are illustrated in Fig. 6. As shown, the ux recovery of MOFs-loaded membranes under visible light irradiation was higher than that of the dark state, due to the photocatalytic reactions inside the pores resulting in the enhanced dissolution of the membrane fouling in water, which in turn improved the water ux aer cleaning under visible light irradiation. 40 The equilibrium uxes of composite nanobous membrane containing 5 wt% UiO-66-NH 2 /TiO 2 was maximum before and aer rising with HCl under visible light irradiation. This behavior indicated the effect of metal organic framework as a porous material and the photocatalytic reaction on the improvement the performance of the metal organic framework-based nanobrous membrane. However, more studies are needed for the reduction of fouling of membranes in the presence photocatalytic reactions.
To investigate the stability of prepared membranes, the Cr(VI) and phenol rejection were investigated for ve cycles using PES/PAN/UiO-66-NH 2 /TiO 2 5% nanobrous membrane under visible light irradiation (Fig. 6e). As shown, the removal efficiencies of Cr(VI) and phenol did not signicantly change even aer ve cycles which demonstrated the stability of the membranes for industrial applications in the future.

Conflicts of interest
There are no conicts to declare.